* Handle relative includes
We change to the directory of the given config file before parsing.
This allows us to handle relative includes.
TODO: Maybe improve the name of the change_dir() function.
* Fix unused result warning
* Add `relative_to` parameter to expand()
If the path is relative, we resolve it by prepending
dirname(config) to the path.
Add dirname() - Returns the parent directory of the file or an empty
string.
* Resolve relative paths
Handle paths relative to the current file being parsed.
* Remove unneeded change_dir()
* Fix expand()
`is_absolute` is calculated after we expand the path.
`relative_to` must be a directory.
Add test for expand() with relative paths
* Recalculate `is_absolute` after expanding `path`
* Add more file_util::expand tests
* Add changelog
Co-authored-by: patrick96 <p.ziegler96@gmail.com>